Direct current (DC) electrical circuits generally provide a source of current to downstream devices operating at a specific voltage. The current that is provided to the downstream devices generally falls within an acceptable range which corresponds to the expected load resistance of the devices. Due to variations in the activity or possible failure of one or more of the downstream devices, the load resistance may vary. Variations in the load, or variations in a voltage or current source attached to the load, causing an unexpectedly high current level can cause device failure if the current drawn by the circuit exceeds a threshold amount.
Alarm circuits are commonly used to protect load devices from electrical currents above the rated threshold of the particular load. Circuit protection devices and alarm triggering mechanisms are commonly used in such alarm circuits. The circuit protection devices, such as fuses or circuit breakers, are generally located at an “upstream” location in series with the load, such that an interruption caused by the circuit protection device disrupts the current path through the load circuit. The alarm triggers generate an alarm signal when the circuit protection device interrupts the circuit to indicate the occurrence of such an interruption.
FIGS. 1A-1C illustrate operation of a prior art alarm circuit using a secondary alarming fuse which can be used in a direct current circuit. FIG. 1A shows normal operation of the alarm circuit in which current passes through the primary circuit protection device, shown as the main fuse 10. FIG. 1B shows the current path when the main fuse 10 blows, and indicates that the current shifts to pass through the secondary fuse placed in parallel with the main fuse 10, shown as GMT fuse 12. Because the GMT fuse 12 is selected to have a lower current rating than the main fuse 10, it also soon blows, enabling the alarm signal 14 as shown in FIG. 1C. This configuration has a cost disadvantage, because each time the primary fuse 10 blows, both it and the secondary alarming fuse must be replaced, adding to the maintenance cost of the circuit. Further, additional user-accessible space is required for two fuses. The secondary fuse also creates a potential electrical hazard because the full input voltage is present at the output terminals of the circuit. Also, unprotected GMT fuses potentially eject the metallic fuse portion of the blown fuse from the fuse holder when blown, creating a fire or injury hazard.
FIGS. 2A-2B illustrate operation of a prior art alarm circuit using a monitoring circuit. FIG. 2A shows normal operation of the alarm circuit, in which a monitoring circuit 16 connects in parallel to the main fuse 10. The current passes through the main fuse 10 and load 20. FIG. 2B shows operation of the alarm circuit after the main fuse 10 blows. The current passes through the monitoring circuit 16 and load 20. Therefore, even when the main fuse 10 blows, a current path exists through the monitoring circuit 16 and a voltage appears at the output terminals. Therefore, it can be difficult for monitoring personnel to detect the location or existence of the fault.
FIG. 2C illustrates operation of an alarm circuit where two power feeds are connected to a load equipped with OR-ing diodes for power redundancy. In such a configuration, when a main fuse 10 blows, leakage current flows though the OR-ing diode which in turn prevents the monitoring circuit from detecting the status of the main fuse. As a result, no fuse alarm signal is generated and the load is no longer protected with dual power feeds. In addition, the leakage current creates a potential across the power input terminals of the load preventing on-board voltage sensors from detecting a fault condition. This can result in a catastrophic system failure because there is no warning of a fault condition.
Therefore, improvements are desirable.